The newest measure will now require persons traveling between the United States and Canada, the Caribbean, Bermuda, Mexico and Central and South America to present a valid passport before boarding an airline. The rule was put in place to help deter possible terror suspects and forged travel documents. The new passport rule is not applicable to U.S. citizens traveling within the United States. The second phase of the program requires passports for entry via vehicle, cruise ship and by foot. That will go into effect next year.